1. Field of the Invention
The present invention relates to a method for making trenches in a semiconductor device, and more particularly to a method of increasing the radius of curvature of trench bottom and trench top corner.
2. Related Background Art
There is a trend for high voltage semiconductor device including MOS (metal-oxide-semiconductor) type and IGBT (Insulated Gate Bipolar Transistor) type to use trench cell structure. Compared with planar cell structure, trench cell structure can achieve higher packing density and lower forward saturation voltage. However, trench structure device has two problems. Firstly, the electric field tends to concentrate on the trench bottom, negatively impacting device's HTRB (high temperature reverse bias) reliability performance. Secondly, trench top corners have a sharp, almost square shape, causing gate leakage problems. So there is a need to increase the radius of curvature of both trench bottom and trench top corner.
In an attempt to increase the radius of curvature for trench bottom, an isotropic etching is performed to round the trench bottom corner while the trench sidewall is protected by a previously deposited reaction product as disclosed in U.S. Pat. No. 6,521,538 B2 (which issued to Soga et al. on Feb. 18, 2003), which is incorporated herein by reference and is hereinafter referred to as the '538 patent. However, this method of rounding the trench bottom corner has limited efficiency because the isotropic etching process is difficult to control.
Therefore, instead of isotropic etching process, others have proposed a method using thermal oxidation process to enlarge the size of the trench bottom as disclosed in U.S. Pat. No. 8,659,065 B2 (which issued to Sumitomo et al. on Feb. 25, 2014), which is incorporated herein by reference and is hereinafter referred to as the '065 patent. This method does meet the requirement of using a highly controllable process like thermal oxidation, but this method still has two drawbacks. Firstly it needs two separate silicon trench etching steps, increasing process complexity and the cost of manufacturing. Secondly, the trench top corner still has a sharp, almost square shape.
To overcome the shortcomings, the present invention intends to provide an improved method using thermal oxidation process to increase the radius of curvature of the trench bottom, and at the same time the trench top corner is rounded.
The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings